A quantum key distribution system is configured with a transmitter, a receiver, and an optical fiber that connects the transmitter and the receiver. The transmitter transmits photons to the receiver via the optical fiber (a quantum communication channel). After that, the transmitter and the receiver exchange control data with each other, and share encryption keys. This technology is implemented using the technology generally referred to as quantum key distribution (QKD).
In quantum key distribution, the behavior of the photons follows the uncertainty principle, which is the fundamental principle of quantum mechanics that tapping leads to changes in the state. Because of such a property, when photons transmitted by a transmitter are tapped by an eavesdropper in a quantum communication channel, the state of the photons undergoes a change and the receiver that receives the photons becomes able to get to know that the photons have been tapped by an eavesdropper.
In the conventional quantum key distribution system, sharing of encryption keys among nodes is assumed to be on one-to-one basis. Thus, sharing of encryption keys is performed in pairs of a transmitter that sends photons and a receiver that receives photons. However, in recent years, it has become possible to share, on a one-to-many basis, encryption keys among nodes that are connected via an optical device. In that regard, attention is being given to a quantum access network (QAN) in which a single receiver can share encryption keys with a number of transmitters.
In order to generate an encryption key that is to be shared using quantum key distribution, a key distillation process is performed while the transmitter and the receiver communicate to each other. The key distillation process includes a shifting process for sharing a bit string, which serves as the basis for the encryption key, between the transmitter and the receiver; an error correction process for correcting errors in the shared bit string; and a key compression process (a privacy amplification (PA) process) for removing the information that has leaked to an eavesdropper during the shifting process and the error correction process.
In a one-to-many type of quantum key distribution system including a quantum access network, the key distillation process performed for the purpose of sharing encryption keys between transmitters and receivers is performed in each node (each transmitter and each receiver). In that case, the “many” nodes of the one-to-many communication perform the key distillation process only for themselves, while the “one” node connected to the “many” nodes needs to perform the key distillation process for a plurality of nodes. Consequently, the single node (a transmitter or a receiver) that is connected to a plurality of nodes (receivers or transmitters) needs to perform many processes. Hence, as compared to the plurality of nodes, there occurs a bias in the processing load of the single node.